Conventionally, techniques such that an overcoat layer is formed on the surface of an image on a ticket, a catalogue or a color page of a magazine using a varnish to impart expensive-looking to the image have been used. Particularly, in mercantile field, such a layer is typically formed on a large number of images formed by printing such as screen printing. Although these images typically have high image area ratios, the images having such a layer are clear and have expensive looking because the applied varnish has good compatibility with inks used for screen printing.
Recently there is a need for frequently changing or updating information to be printed. Since screen printing performs printing after preparing an original plate, screen printing cannot fulfill the need because a profit is hardly produced thereby. Therefore, so-called on-demand printing has been performed therefor.
Devices using electrophotography and inkjet recording methods are typically used for on-demand printing. Since it takes time before drying an ink image formed by inkjet recording, it is difficult for inkjet recording to quickly produce a large number of images although inkjet recording can be used for producing a small number of images. In addition, when an ink image is formed on a paper sheet and then dried, the paper sheet is typically expanded and then contracted, and the thickness of some parts of the paper slightly changes, thereby causing a stacking problem in that prints cannot be stacked orderly. Therefore, electrophotographic image forming methods using toner are mainly used for on-demand printing now. Electrophotographic image forming methods typically include charging a photoreceptor; irradiating the charged photoreceptor to form an electrostatic latent image thereon; developing the electrostatic latent image with toner to form a toner image on the photoreceptor; transferring the toner image onto a recording medium such as paper sheets; and fixing the toner image to the recording medium upon application of heat thereto.
In attempting to form such an overcoat layer as mentioned above in electrophotographic image forming methods, a technique is proposed which uses an aqueous overcoat layer composition liquid, which includes water as a main component without including ammonia and which has a low static surface tension, for forming an overcoat layer on images on which an oil used is applied by a fixing member in a fixing process.
In addition, a resin layer forming device, and an image forming apparatus equipped with the resin layer forming device are proposed which form a silicone resin layer is formed on a recorded image to protect the image while waterproofing and glossing the image.
Further, in attempting to efficiently perform high-mix low-volume printing using electrophotographic image forming methods, a printing method in which a varnish is applied on a toner image formed on a metal container to protect the toner image while glossing the image is proposed.
These methods are preferable when forming an overcoat layer on an image formed by electrophotography.
In fixing devices of conventional electrophotographic image forming apparatuses, a large amount of silicone oil is applied on the surface of a fixing roller to improve the releasability of the fixing roller from a toner image on a recording medium. However, the releasability of a surface portion of a fixing roller coated with a silicone oil is largely different from that of a surface portion of the fixing roller which is not coated with the silicone oil, and if the fixing roller has a surface portion which is not coated with the silicone oil, an image having uneven glossiness (i.e., an image having linear non-glossy portions) is formed. If such images are formed in commercial printing, the percentage of defective prints seriously increases, thereby increasing the manufacturing costs. In addition, when such a silicone oil is adhered to a floor, the floor becomes very slithery, and in addition it is difficult to perfectly remove the silicone oil adhered to the floor. Therefore, when such a silicone oil is supplied to a fixing device or a maintenance operation is performed on the fixing device, the person in charge has to perform the supplying operation and the maintenance operation with extreme caution. Therefore, the persons in charge dislike the operations terribly.
Recently, instead of such fixing methods using a silicone oil, image forming methods using a so-called oil-less fixing method have been used. In such image forming methods, a toner including a wax is used for forming a toner image, and when the toner image is fixed by a fixing roller upon application of heat thereto, the wax is exuded from the toner to improve the releasability of the toner image from the fixing roller.
In such oil-less fixing, the more the amount of wax present between a fixing roller and a toner image, the better the releasability of the fixing roller from the toner image. Therefore, a wax having a low melting point is typically added to the toner while increasing the added amount of the wax as much as possible, and the fixing conditions such as pressure of the fixing roller, fixing temperature, and fixing time are properly adjusted so that the wax in the toner is easily exuded therefrom in the fixing process.
When an overcoat layer is formed on an image subjected to such oil-less fixing, the following problems (1) and (2) tend to be caused:
(1) The wax present on the image repels a coating liquid including the overcoat layer composition, and therefore the overcoat layer becomes very thin on the image area.
(2) The adhesiveness of the overcoat layer, which is crosslinked, with the image deteriorates, thereby causing a problem in that when the image area is rubbed or bent, the overcoat layer is released from the image.
For these reasons, the inventors recognized that there is a need for an image forming method which includes forming an overcoat layer on a toner image fixed by oil-less fixing and which can produce a clear image having expensive-looking and good rub-resistance.